In direct, as opposed to transfer, electrography or xerography, a recording medium such as paper is advanced past a succession of processing stations. First, an electrostatic charge is impressed on the photoconductive surface of the paper. Then, the paper is exposed to a light image of an original document or record with the result that those areas of the paper that receive light are discharged while the remaining areas of the paper which are not illuminated retain their charge so that a electrostatic latent image corresponding to the original document is impressed on the paper. That image is then developed by applying liquid toner to the paper. The toner contains resin binder and black or colored pigment or particles suspended in a carrier liquid. The particles carry an electric charge that is oposite that of the paper so that the particles migrate and adhere to only to those areas on the paper which still carry an electric charge whereby the latent image on the paper is reduced to visible form. Finally, the developed image on the paper is fixed by passing the paper through a dryer which heats the binder thereby bonding the toner to the paper, creating a permanent copy of the original document or record.
Similar image recorders make reproductions on dielectric recording media. In those, only the aforementioned steps of charging and toning take place in the recording apparatus.
We are concerned here specifically with the toner applicator at the toning station of such recording apparatus which distributes liquid toner across the charged exposure surface of the recording medium, and which is critical to the creation of high quality reproductions of an original document or record. We will describe the invention as incorporated into recording apparatus using a photosensitive recording medium. It should be understood, however, that it is equally applicable for use in recorders such as those marketed by Sanders Associates, Inc. under the designation CALCOMP which make reproductions on dielectric recording media.
Conventional applicators have a working or bearing surface that contacts the photoconductive surface of the medium. A multiplicity of small passages lead from locations on that bearing surface to a source of liquid toner. The liquid is pumped through the passages forming a film between the head and the recording medium. Some heads also include other passages maintained at a negative pressure for removing excess toner from the medium surface. Examples of such toner applicators are disclosed in U.S. Pat. Nos. 3,929,099; 3,727,578; 4,133,906 and 4,281,620.
While certain existing applicators do fulfill their basic function of distributing toner over the charged surface areas of the recording media, they have serious drawbacks which restrict their ability to develop electrostatic images of high quality at reasonable cost. More particularly, in some prior applicators, the liquid toner is not flowed equally to all regions of the charged exposure surface of the medium with the result that dark areas of the image developed on the medium are nonuniform or blotchy. Some toning heads do achieve uniform flow of liquid over the medium. However, the toner reaching certain portions of the medium surface is partially or totally depleted of toner particles so that the image developed on the medium has regions which are gray instead of black. Some conventional electrographic recorders suffer because the toner is not maintained in contact with the charged surface of the recording medium long enough for an adequate number of toner particles to reach and adhere to the medium surface, again creating a copy having poor contrast. This may be due to the fact that the electrostatic field intensity in the gap between the head and the recording medium is too weak or the residence time of fresh or undepleted toner flowing in that gap is too short. Those applicators which do develop reasonably high contrast, high quality electrographic images on the medium tend to be relatively complicated machined structures which are difficult to manufacture and therefore relatively expensive to make and maintain.